The invention relates to commutation control methods and circuitry for electric motors including at least three phases energized in a cyclic sequence having a plurality of states, wherein during each state, two of the phases are energized and the third phase is unenergized.
In a plural phase electric motor, the phases are energized to create a rotating magnetic field, which the rotor will follow due to the torque produced thereon. To sustain rotation of the rotor, the phase windings are energized in a given sequence having a plurality of states. The change from one state or to the next state is called commutation. Commutation ensures continued rotation of the magnetic field, and hence continued rotation of the rotor. It is desirable to know the position of the rotor, in order to select the proper commutation timing and energization state of the various windings, and hence provide the optimum magnetic field pattern for producing optimum torque on the rotor. When the rotor passes a given position, it is desired to commutate to the next state in the energization sequence of the phase windings, to continue to apply torque to the rotor.
In the control of variable speed electric motors, it is desirable to have feedback of motor speed. This feedback typically consists of an external sensor that connects to the motor shaft and directly reads the rotor information. This external sensor adds an undesirable cost to the motor control, and requires additional wiring to the motor. The burden of these extra wires is compounded in applications such as air conditioning compressors, where the motor is hermetically sealed. It is therefore desired in various applications to eliminate external sensors and extra wires.
Various sensorless motor control systems are known in the prior art. In one method, back emf of an unenergized phase is sensed to control commutation to the next state. In another method, changing reluctance of the unenergized phase is sensed as a function of rotor position, to control commutation to the next state. In the latter method, the unenergized phase winding is pulsed with a test current to determine changing reluctance.
In the present invention, regeneration current is sensed from the unenergized phase, and the motor is commutated to the next state in response to such regeneration current. In the preferred embodiment, bus regeneration current is sensed with all commutation switches off. In an alternate embodiment, individual phase regeneration current is sensed.